Define Cross-Origin-Resource-Policy response header

This header makes it easier for sites to block unwanted "no-cors" cross-origin requests.

Tests: ...

Fixes #687.

fetch.bs

@@ -2455,6 +2455,64 @@ run these steps:</p>
 </ol>
 
 
+<h3 id=cross-origin-resource-policy-header>`<code>Cross-Origin-Resource-Policy</code>` header</h3>
+
+<p>The
+`<dfn export http-header id=http-cross-origin-resource-policy><code>Cross-Origin-Resource-Policy</code></dfn>`
+response <a for=/>header</a> can be used to require checking a <a for=/>request</a>'s
+<a for=request>current url</a>'s <a for=url>origin</a> against a <a for=/>request</a>'s
+<a for=request>origin</a> when <a for=/>request</a>'s <a for=request>mode</a> is
+"<code>no-cors</code>".
+
+<p>Its <a for=header>value</a> <a>ABNF</a>:
+
+<pre>
+Cross-Origin-Resource-Policy     = %x73.61.6D.65 / %x73.61.6D.65.2D.73.69.74.65 ; "same" / "same-site"; case-sensitive</pre>
+
+<p>To perform a <dfn>cross-origin resource policy check</dfn>, given a <var>request</var> and
+<var>response</var>, run these steps:</p>
+
+<ol>
+ <li><p>If <var>request</var>'s <a for=request>mode</a> is not "<code>no-cors</code>", then return
+ <b>allowed</b>.
+
+ <li>
+  <p>If <var>request</var>'s <a for=request>origin</a> is <a>same origin</a> with
+  <var>request</var>'s <a for=request>current url</a>'s <a for=url>origin</a>, then return
+  <b>allowed</b>.
+
+  <p class=note>A cross-origin response redirecting to a same or same-site resource with the
+  `<a http-header><code>Cross-Origin-Resource-Policy</code></a>` header specified does not affect
+  anything.
+  <!-- We could make this have an effect if we fix https://github.com/whatwg/fetch/pull/594 first,
+       but even then we normally do not let this have any effect for "no-cors" so it would be
+       somewhat inconsistent if it did here, but might still be better... -->
+
+ <li><p>Let <var>policy</var> be the <a>combined value</a> with
+ `<a http-header><code>Cross-Origin-Resource-Policy</code></a>` and <var>response</var>'s
+ <a for=response>header list</a>.
+
+ <li><p>If <var>policy</var> is `<code>same</code>`, then return <b>blocked</b>.
+
+ <li>
+  <p>If <var>policy</var> is `<code>same-site</code>` and neither of the following is true
+
+  <ul class=brief>
+   <li><p><var>request</var>'s <a for=request>origin</a>'s <a for=origin>host</a>
+   <a>is a registrable domain suffix of or is equal to</a> <var>request</var>'s
+   <a for=request>current url</a>'s <a for=url>host</a>
+
+   <li><p><var>request</var>'s <a for=request>current url</a>'s <a for=url>host</a>
+   <a>is a registrable domain suffix of or is equal to</a> <var>request</var>'s
+   <a for=request>origin</a>'s <a for=origin>host</a>
+  </ul>
+
+  <p>then return <b>blocked</b>
+
+ <li><p>Return <b>allowed</b>.
+</ol>
+
+
 
 <h2 id=fetching>Fetching</h2>
 
@@ -3751,6 +3809,9 @@ Range Requests</cite>. [[HTTP-RANGE]] However, this is not widely supported by b
     </ol>
   </ol>
 
+ <li><p>If the <a>cross-origin resource policy check</a> with <var>request</var> and
+ <var>response</var> returns <b>blocked</b>, then return a <a>network error</a>.
+
  <li>
   <p>If <var>response</var>'s <a for=response>status</a> is <code>401</code>, <i>CORS flag</i>
   is unset, <i>credentials flag</i> is set, and <var>request</var>'s <a for=request>window</a> is an
@@ -6390,6 +6451,7 @@ Jeffrey Yasskin,
 Jesse M. Heines,
 Jinho Bang,
 Jochen Eisinger,
+John Wilander,
 Jonas Sicking,
 Jonathan Kingston,
 Jonathan Watt,